1. Field of the Invention
The present invention generally relates to magneto-resistive elements, magnetic heads and magnetic storage apparatuses, and more particularly to a magneto-resistive element having a Current Perpendicular to Plane (CPP) structure for causing a sense current to flow in a direction in which layers are stacked by use of a so-called spin valve layer or a magnetic tunneling junction layer, a magnetic head employing such a magneto-resistive element for reproducing information from a magnetic recording medium, and a magnetic storage apparatus employing such a magneto-resistive element.
2. Description of the Related Art
Conventionally, magneto-resistive elements are used as reproducing elements for magnetic heads that reproduce recorded information from a magnetic recording medium in a magnetic storage apparatus. Magneto-resistive elements that are provided with a spin valve layer having a high magnetic field sensitivity are popularly used in order to cope with high recording densities. The spin valve layer is formed by two ferromagnetic layers, namely, a pinned magnetization layer having a magnetization direction fixed by an antiferromagnetic layer and a free magnetization layer having a magnetization direction that changes depending on a leakage magnetic field from the magnetic recording medium.
A Current-In-Plane (CIP) structure that causes the sense current to flow in an in-plane direction of the spin valve layer has mainly been used in the past. However, in order to further improve the high recording density, it is necessary to increase a linear recording density and a track density, and for this reason, an area of the magnetic recording medium covering 1 bit consequently decreases. As a result, a leakage magnetic field from the magnetic recording medium, that is, a decrease in a signal magnetic field occurs. Furthermore, in order to reproduce a high-density signal by the reproducing element, it is necessary to reduce a width and a height of the reproducing element. According to the CIP structure, the sense current must be set small in order to prevent performance deterioration due to migration and the like. For this reason, a magnitude of a variation in a detected magneto-resistance decreases, and a reproduced output becomes small as the recording density further increases, thereby making detection of fine signal magnetic fields difficult.
A Current Perpendicular to Plane (CPP) structure has been proposed as a next-generation reproducing element, and active research is being made on the CPP structure. The CPP structure causes the sense current to flow in the direction in which the layers of the spin valve layer are stacked.
FIG. 1 is a cross sectional view showing an example of a conventional magneto-resistive element employing the CPP structure. In FIG. 1, when a sense current I flows in a direction in which layers of a spin valve layer 100 employing the CPP structure are stacked, the magneto-resistance varies depending on relative directions of the magnetization of a free magnetization layer 108 and the magnetization of a ferromagnetic layer 104 of a pinned magnetization layer 105. A signal output of the magneto-resistive element is detected as a voltage variation across both ends of the spin valve layer 100. The pinned magnetization layer 105 has a stacked ferri structure in which two ferromagnetic layers 102 and 104 of a CIP structure are antiferromagnetically coupled via a nonmagnetic coupling layer 103. Since the magnetizations of the two ferromagnetic layers 102 and 104 are mutually antiparallel, the magnitude of the magnetization of the stacked ferri structure becomes small, to reduce the diamagnetism. Thus, an exchange coupling between the stacked ferri structure and the antiferromagnetic layer 101 can be increased while suppressing a net magnetization, so as to positively fix (or pin) the magnetization direction of the pinned magnetization layer 105.
However, in the case of the CPP structure, the spin valve layer 100 through which the sense current I flows has a small thickness. For this reason, an element resistance of the CPP structure is lower than that of the CIP structure, and as a result, there is a problem in that a sufficiently large signal output cannot be obtained by the CPP structure.
In order to obtain a sufficiently large signal output, it is necessary to increase a product of an amount of variation ΔR of the magneto-resistance due to a change in an external magnetic field and an area A of the spin valve layer 100, that is, an amount of variation ΔRA of the magneto-resistance per unit area. For this purpose, search is being made for suitable materials that may be used for the free magnetization layer 108 that causes magneto-resistance and the ferromagnetic layer 104 on the side of the free magnetization layer 108 forming the pinned magnetization layer 105. However, it is difficult to developed new suitable materials, and there are problems in that there is a limit to selecting the materials, and that a sufficiently large signal output cannot be obtained.
Particularly in the case of the CPP structure, the sense current flows in the direction in which the layers of the stacked ferri structure are stacked, and a resistance caused by bulk scattering is determined by a relationship of an electron spin direction and the magnetization direction. Due to the electrons passing through the two ferromagnetic layers 102 and 104 of the pinned magnetization layer 105 having antiparallel magnetization directions, a difference between the two resistances decrease, and there is a problem in that the signal output is further decreased thereby.